Cephalization is the process in animals by which nervous and sensory tissues become concentrated in the "head." The evolution of a head allows scientists to distinguish between the head end, or anterior end of an animal's body, and the opposite end, the posterior. Although cephalization is associated primarily with bilaterally symmetrical species, even some of the more primitive, radially symmetrical animals show some degree of cephalization.

Cephalization evolved several times within the animal kingdom, suggesting that it offers certain inherent advantages. In particular, with the evolution of cephalization, the anterior end of the animal became most likely to first encounter food, predators, and other important features of the external environment. Because brain and sense organs are also concentrated in the anterior end, the organism is well prepared to deal with these features.

Cephalization in the Animal Kingdom

Even hydras, which are primitive, radially symmetrical cnidarians, show some degree...